As a communication standard adopted for a basic transmission line of a backbone circuit etc. and other transmission lines etc., there is an OTN (Optical Transport Network) recommended in the ITU-T (International Telecommunication Union Telecommunication standardization sector). In the OTN, a plurality of circuits (for example, a current circuit and a standby circuit) set for a transmission line are switched to improve the reliability and operability of a transmission line. A system of switching a plurality of circuits may be a 1+1 switch, a 1:n switch, etc. Switching a plurality of circuits is performed by each transmission apparatus referring to the APS (Automatic Protection Switching) byte (otherwise, the APS channel).
On the other hand, in the OTN, in addition to the concept of a segment (transmission section) such as a path and a section adopted in the SDH (Synchronous Digital Hierarchy), the SONET (Synchronous Optical NETwork), etc., a new concept of a segment, that is, a TCM (Tandem Connection Monitoring), is introduced. The TCM may be freely set for the segment of an arbitrary position on the transmission line or arbitrary length. That is, by introducing the TCM, a specific segment may be set for each network provider and user. Therefore, a fault generation state, a circuit quality, etc. may be monitored for the specific segment for each network provider and user.